The value of a for which the equations $3x-y+az=1, 2x+y+z=2, x+2y-az=-1$ fail to have unique solution is __________ 

$3x-y+az=1\_\_\_\_\_\_\left(1\right)$

$2x+y+z=2-------\left(2\right)$

$x+2y-az=-1------\left(3\right)$

Eq,$\left(1\right)+\left(2\right)\Rightarrow 5x+\left(a+1\right)z=3-----\left(4\right)$

$2\times \left(1\right)+\left(3\right)\Rightarrow 7x+az=1------\left(5\right)$

Given system fail to have  unique solution.

$\Rightarrow$  The system has no solution or infinitely many solutions

$\begin{array}{l}\Rightarrow \frac{5}{7}=\frac{a+1}{a}\ne \frac{3}{1}\\ \Rightarrow 5a=7a+7\mid \\ \Rightarrow 2a=-7\\ \Rightarrow a=-\frac{7}{2}\end{array}$